Technique for etch polishing single rare earth crystal synthetic garnet

ABSTRACT

A technique for etch polishing single crystal synthetic garnet involves etch polishing the crystal in hot phosphoric acid saturated with steam. The described technique significantly enhances reproducibility and results in crystals manifesting an amplitude of surface undulations of + OR - 0.25 microns or less with a frequency of 100 or greater.

United States Patent Miller Aug. 28, 1973 TECHNIQUE FOR ETCH POLISHING[56] References Cited SINGLE RARE EARTH CRYSTAL UNITED STATES PATENTSSYNTHETIC GARNET 3,080,320 3/1963 Vickery 252/625 Inventor: DavidChristopher Miller,

Millington, NJ.

Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, Berkeley Heights, NJ.

Filed: Dec. 9, 1971 Appl. No.: 206,529

US. Cl 156/2, 156/20, 252/625 Int. Cl. H011 7/50, H011 7/62 PrimaryExaminer-Jacob l-l. Steinberg Attorney-W. L. Keefauver et a1.

[57] ABSTRACT A technique for etch polishing single crystal syntheticgarnet involves etch polishing the crystal in hot phosphoric acidsaturated with steam. The described technique significantly enhancesreproducibility and results in crystals manifesting an amplitude ofsurface undulations of $0.25 microns or less with a frequency of 100 orgreater.

8 Claims, No Drawings TECHNIQUE FOR ETCI-I POLISHING SINGLE RARE EARTHCRYSTAL SYNTHETIC GARNET This invention relates to a technique foretching single crystal synthetic garnet. More particularly, the presentinvention relates to a technique for etch polishing single crystalsynthetic garnet in phosphoric acid.

DESCRIPTION OF THE PRIOR ART In recent years, the most promisingcommercial applications of synthetic garnet have been in microwavedevices and in bubble domain devices. In each of these cases, the garnetselected should desirably be flat within 1 micron, smooth and highlypolished. Heretofore, this end has been attained by conventionalmechanical polishing techniques which are inordinately time consuming,economically expensive and not readily adaptable to manufacturingtechniques. Workers in the art have utilized heated phosphoric acid toetch a plurality of single crystal oxides but limitations have beenimposed by precipitate formation. and the transition of the acid tounsatisfactory etchants. Unfortunately, this procedure when applied tosynthetic garnet has proven to be unsatisfactory in that theaforementioned difficulties are greatly increased. Additionally, studieshave revealed that such processes are not reproducible and, therefore,unsatisfactory for commercial applications.

SUMMARY OF THE INVENTION In accordance with the present invention, theseprior art limitations are effectively obviated by a novel techniqueutilizing steam saturated phosphoric acid as the etchant. Briefly, theinventive technique involves etch polishing the crystal in hotphosphoric acid saturated with steam and subsequently quenching thecrystal in oil. Synthetic garnet treated in the foregoing mannermanifests an amplitude of surface undulations of 10.25 micron with afrequency of 100 or greater.

DETAILED DESCRIPTION OF THE INVENTION I The synthetic garnet materialsconsidered here can be represented by the general formula M,Me,,0 whereOis oxygen, Me is a trivalent metal, and M is yttrium or one of the rareearth elements of atomic number between 62 and 71 or a mixture of theserare earth elements with each other. Me may be trivalent iron or one ofthe elements selected from among gallium, aluminum, scandium, ormixtures thereof.

In order to aid in the understanding of the present invention, anoutline of the procedure employed in the etching of synthetic garnetwill now be given.

The first step in the practice of the present invention optionallyinvolves pre-etching the garnet of interest with phosphoric acid. Thegarnet materials described above are conveniently obtained fromcommercial sources or are grown in accordance with standard techniques.The as-grown crystals are cut in the desired size and then pre-etched inphosphoric acid at.a temperature within the range of 155 180C. Thepurpose of this pre-etching step is to reduce the statistical likelihoodof precipitate formation and to dissolve the highly damaged surfacelayer with its accompanying grit. Studies have revealed that the use oftemperatures less than the noted 155 minimum will not yield asatisfactory etch rate, whereas the-use of temperatures in excess of180C result in excessive pitting of crystal surfaces. Etching inaccordance with this schedule is continued for a time period rangingfrom 5 to 10 minutes, the minimum and maximum being dictated by the samecriteria noted above with respect to temperature.

The next step in the practice of the present invention involvesimmersing the pre-etched crystal in phosphoric acid which is heated to atemperature within the range of 250 450C and to which steam heat to atemperature within the range of 200C has been added in an amountsufficient to saturate the acid, thereby establishing a moisture levelwhich can be conveniently maintained throughout a plurality ofprocessing sequences. Immersion is continued for a time period rangingfrom 2 to 5 minutes, the duration of immersion being dictated byconsiderations relating to the amount of material to be removed.

It has been determined that the etch character is a function of thetemperature as well as the moisture content of the etching bath. Attemperatures in excess of the noted 450 maximum, crucible attack hasbeen noted. The use of temperatures less than 250C results in a highfrequency of undulations. However, processing in accordance with theforegoing schedule of pa rameters results in the formation ofhigh-quality polished surfaces with a stabilized etch rate and characterand a concurrent high degree of reproducibility. The rate of removal ofmaterial in the processing sequences described hereinis approximately ofthe order of 1 mil per minute.

The phosphoric acid selected for use in the practice of the presentinvention is orthophosphoric acid having a concentration of 85 percent,such being readily available from commercial sources.

Following, the etched crystal is quenched in oil and subsequentlycleansed with any conventional detergent. The particular oil selected isnot critical, mechanical pump oil being found particularly suitable forthis purpose. In order to obviate the likelihood of cracking during thequenching operation, it is convenient to heat the oil, temperatures ofthe order of 60C being found suitable for this purpose.

Examples of the application of the present invention are set forthbelow. They are intended merely as illustration, and it is to beappreciated that the methods described may be varied by one skilled inthe art without departing from the spirit and scope of the presentinvention.

The examples are in tabular form for convenience and brevity. Eachexample in the table is to be considered separately, since each set ofdata was obtained in a separate process. The procedure followed in eachof these examples is as follows:

The synthetic garnet crystal of interest, obtained by standard growthprocesses, was initially cut in discs approximately 2 centimeters indiameter by 1 millimeter in thickness and placed in a platinum crucible.Then, orthophosphoric acid (85 percent concentration) was introducedinto the crucible and heating initiated at a temperature of C for 5minutes. Then, the preetched crystal was heated in the presence of thephosphoric acid .to 390C and steam heated to 200C introduced into thephosphoric acid to effect saturation thereof, saturation being indicatedby the presence of an equilibrium, that is, the volume of steam enteringsolution is equivalent to the volume evolved from the acid. Immersion inthis solution was continued for 2 minutes. Next, the crystal was removedfrom the crucible and immersed in mechanical pump oil heated to a netcrystal evidenced precipitate formation, surface temperature of 60C.Finally, the crystal was cleansed cracking and lack of reproducibilityfrom step to step with a conventional detergent and potassium hydroxideas the acid deteriorates. and the quality of the solution etched surfacestudied. What is claimed is:

1. Technique for etch polishing single crystal syng a g gg iggi theticgarnet of the general formula M Me O where Talysurf Surface 0 is oxygen,Me a trivalent metal, and M is yttrium Example l Measurements or one ofthe rare earth elements of atomic number be- 1 Gadolinium Gallium GamerAmplitude f urf undulations tween 62 and 71 or mixtures thereof whichcomprises of less than $015 micro" will 10 (a) etch polishing thecrystal in phosphoric acid satua frequency of 100 or greater 2 YttriumAluminum rated with steam for a time period ranging from 2-5 GarnetAmplitude of Surface undulations minutes, the temperature of thephosphoric acid being g eate within the range of 250-450C, and (b)quenching the 3 Holmium Gallium etched polished crystal in oil.

4 Dyspwsh'l l5 2. Technique in accordance with claim 1 wherein GalliumGarnet said garnet is pre-etched in phosphoric acid at a tem- 5 gfigiperature within the range of l55-l80C for a time pe- 6 S -i Gadoliniumriod ranging from 5-10 minutes prior to etch polishing.

7 S3L2L gg2 g 3. Technique in accordance with claim 1 wherein the GamumGame steam is heated to a temperature within the range of 8 GadoliniumScandium lO5-200C.

9 r. 4. Technique in accordance with claim 1 wherein Garnet quenching iseffected in oil heated to a temperature of g ggfl gg fgf the order of60C.

l1 N odymium Scandium 5. Technique in accordance with claim 2 wherein l2gzgiggfix said pre-etching is conducted at a temperature of ScandiumAluminum Garnet 165C and the etch polishing is conducted at 390C.

6. Technique in accordance with claim 4 wherein Additionally, thecrystal of Example 2 as etched could said single crystal garnet isgadolinium gallium garnet. not be distinguished from a mechanicallypolished sur- 7. Technique in accordance with claim 4 wherein face, Forcomparative purposes, the procedure desaid single crystal garnet .isyttrium aluminum garnet. scribed above was reiterated with the exceptionthat 8. Technique in accordance with claim 4 wherein steam was not addedto the heated phosphoric acid in said single crystal garnet is holmiumgallium garnet. the etching step. The resultant gadolinium gallium gar-I

2. Technique in accordance with claim 1 wherein said garnet ispre-etched in phosphoric acid at a temperature within the range of155*-180*C for a time period ranging from 5-10 minutes prior to etchpolishing.
 3. Technique in accordance with claim 1 wherein the steam isheated to a temperature within the range of 105*-200*C.
 4. Technique inaccordance with claim 1 wherein quenching is effected in oil heated to atemperature of the order of 60*C.
 5. Technique in accordance with claim2 wherein said pre-etching is conducted at a temperature of 165*C andthe etch polishing is conducted at 390*C.
 6. Technique in accordancewith claim 4 wherein said single crystal garnet is gadolinium galliumgarnet.
 7. Technique in accordance with claim 4 wherein said singlecrystal garnet is yttrium aluminum garnet.
 8. Technique in accordancewith claim 4 wherein said single crystal garnet is holmium galliumgarnet.